Multilayer electrostatic copyboard and light panel



Nov. 25, 1969 .s, F. BARNETT 0,

MULTILAYER ELECTROSTATIC COPYBOARD AND LIGHT PANEL Filed July 28, 19s? INVENTOR ear 5 BARNETT fimmz A TTORNEVS.

United States Patent M US. Cl. 35575 11 Claims ABSTRACT OF THE DISCLOSURE An electrostatic copyboard, preferably of the backlighted type, in which a high voltage DC. potential is applied to a conductive surface having a dielectric insulative overlay against Which articles, such as paper, cardboard, and other conductive and non-conductive components are held by electrostatic forces. In the present board or panel, the insulative overlay includes a plurality of dielectric layers which are superimposed upon each other in face-to-face relationship but spaced from each other by approximately 100 atomic spacings.

BRIEF SUMMARY OF THE INVENTION This invention relates to improvements in electrostatic copyboards and light panels which retain articles and components in position for making composite layouts, for photography, and for tracing purposes, as well as other related operations.

In prior US. Patent No. 3,359,469, entitled Electrostatic Pinning Method and Copyboard, there is shown a device for pinning articles by means of electrostatic forces. The said application demonstrates a system for countering diminution of electrostatic holding forces which result from repeated placement of the articles by reversing the polarity of the voltage applied to the board. This prior application also teaches a manner for reduction of shock propensities in such a board without diminishing holding power by utilizing a semiconductive intermediate layer having a volume resistivity in the range of ohm centimeters.

In prior patent application Ser. No. 552,483, entitled Electrostatic Copyholder and Method for Making Same, there is shown a grid type conductive construction on the distal surface of the dielectric layer to define a multiplicity of condensers for enhancing electrostatic retaining forces for conductive articles.

The present invention is directed to the further improvement of electrostatic holding forces by considering theoretical aspects of potential and geometric characteristics. It can be shown that the force per unit area for an electrostatic copyboard, which is essentially a parallel plate condenser, is ideally:

p is the force per unit area s is the permittivity of free space k is the relative permittivity or the dielectric constant of the dielectric V is the applied voltage, and

t is plate spacing where:

from

Q=CV=kC V which can be logically developed, where Q is the total charge, C is the capacitance, and C is the capacitance with air dielectric.

3,480,364 Patented Nov. 25, 1969 ice The electric fields Within the electrostatic copyboar can be given by the general expression:

D=E E+P which relates the fields to the charges which produce them.

where:

D is the displacement representing the total charge density D= T=ke E E is the applied electric field, and P is the polarization field The foregoing equation indicates simply that the total charge density is equal to the sum of the free charge density and the bound or polarization charge density. Also it can be shown that P=(k--1)e E=Nu or the bound charge per unit area is equal to the dipole moment per unit volume in the dielectric.

where:

N is the number of dipoles per unit volume, and u is the dipole moment (charge X separation).

The average dipole moment for each elementary particle is proportional to the immediate or local electric field, E. Thus,

u=&E'

where & is the polarizability factor comprising e+ a+ d+ l the subscripts:

& is electronic polarizability & is atomic polarizability & is rotational polarizability, and & is interfacial polarizability so that P=N&E

where E is shown to consist of E'==E+E +E and E is the field Within a fictitious spherical cavity produced by the redistribution of dipoles on the spherical surface E is the field due to dipole and charge distribution within the spherical cavity.

It can be shown that E (E/3) (k-l-Z) and that E =0 provided that the dielectric has a completely symmetrical cubic crystal structure or is a completely isotropic amorphous material. Then, the equation is important to us because it permits the calculation of the polarizability factor per mole of dielectric from macroscopic known or measurable quantities for materials where low or moderate dielectric constant is required, such as in opaque and in edge-effect (grid) electrostatic copyboards.

& the electronic or optical polarizability, is an induced dipole moment having all the characteristics of an assembly of dipoles produced by the elastic displacement of electrons in the atom which have natural frequencies equal to or greater than those of visible light. Molecules having a large dipole moment are described as polar molecules. An example of this large dipole moment is in ionic bonding, such as NaCl ionic crystal forms in which a negative charge is separated from a positive charge to form a true dipole.

Under the influence of an applied field, the polarization of a polar substance will change by virtue of two mechanisms: (1) the field may cause the atoms to be displaced with respect to each other and thus alter the distance between them with resultant change in dipole moment. This mechanism is atomic polarizability, &,,, (u=qd, where q is the charge on one component and d is the distance between components); and (2) the molecule as a whole may rotate about its axis of symmetry so that the dipole aligns itself with the field. This is called orientational polarizability 8r In a real solid, there inevitably exists a large number of defects such as lattice vacancies, impurity centers, dislocations, gross physical interfaces, etc. Free charge carriers migrating through the crystal under the influence of the applied field are trapped by or piled up against these defects. It can be shown that a volume resistivity as low as 10 ohm centimeters is adequate to provide a free charge carrier density to obtain maximum population of the available traps. The effect is the creation of a localized accumulation of charge which will induce its image charge on an electrode to give rise to a dipole moment. This constitutes a separate mechanism of polarization in the substance called interfacial polarizability, &

Without resorting to the complete derivation, it can be shown from the foregoing analysis that the holding force of the copyboard, as represented by the force per unit area, 12, is proportional to the charge density, to the dielectric constant, K, of the dielectric layers, and to the polarizability, &.

It is therefore an object of this invention to provide an improved electrostatic copyboard and light panel for holding objects.

Another object of this invention is to improve the electrostatic holding power of such copyboards by increasing the total charge density, a, at the surface where the objects are to be detachably secured.

Still another object of this invention is to provide a means for increasing the bound or polarization charge density, P, at the surface of electrostatic copyboards for a given applied voltage.

Yet another object of this invention is to provide an electrostatic copyboard having large polarizability factor (8:) characteristics.

A particular object of this invention is to improve the interfacial polarizability factor (& characteristics of the dielectric in electrostatic copyboards.

Yet a further object of this invention to provide means for taking maximum advantage of trapping charges in electrostatic copyboards.

A yet still further object of this invention is to utilize a multilayer dielectric design in an electrostatic copyboard as an effective manner of trapping charges.

Another object of this invention is to provide a multilayer dielectric copyboard wtih interfacial isolation.

Other objects of my invention are to provide an improved device of the character described which is easily and economically produced, sturdy in construction, and highly efficient and effective in operation.

DESCRIPTION OF FIGURES With the above and related objects in view, this invention consists of the details of construction and combination of parts as will be more fully understood from the following detailed description when read in conjunction with the accompanying drawing in which:

FIGURE 1 is perspective view of a multilayered electrostatic copyboard embodying this invention.

FIGURE 2 is an exploded perspective view thereof.

FIGURE 3 is an enlarged partial sectional view taken along lines 33 of FIGURE 1.

4 DETAILED DESCRIPTION Referring now in greater detail to the drawing in which similar reference characters refer to similar parts, this electrostatic copyboard invention comprises a frame, generally designated as A, a conductive surface member, generally designated as B, a plurality of thin dielectric layers C1, C2, and C3 superimposed in face-to-face disposition with said conductive surface member B but not in intimate contact with each other, and a high voltage DC power supply D whose high side is coupled to the conductive surface member.

The frame A includes a table or desk like member 12 which is of an appropriate size and adapted to sit on the floor in the manner of a light table. The upper portion of the table 12 has a recessed portion 14 into which are mounted a plurality of lamps 16, such as fluorescent bulbs. A transparent or translucent glass plate 18 covers the light box 14 and defines a base for the instant copyboard.

The conductive surface member B may be a metallic screen 20, for example 30 mesh, of 6.5 mil wire, which will define a substantially continuous surface plane while at the same time permit the transmission of light from the lamps 16 through the interstices of the screen. The screen B is secured to the face of the glass plate 18 by a border or margin 22 of pressure sensitive adhesive tape having both faces adhesive coated. The border 22 is formed by adhering strips of the double faced adhesive first to the margins of the glass plate 18. The adhesive tape 22 has a base of a good insulation material, such as 3 mil Mylar polyester. The screen 20 has a length and width larger than the opening defined by the border but smaller than the external dimensions thereof. Accordingly the screen 20 adheres to border 22 but does not extend to the external edges. A filler border 24 of 10 mil polyester Mylar is laid over the adhesive border 22 in the annular zone surrounding the marginal edges of the screen 20. The filler strip 24 acts both to level and as a spacer to prevent areas of high voltage being exposed. Next a second border of double faced adhesive tape 26 is laid over the screen 20 and filler strip 24. It is also to be noted that a transparent metallic coating 25, shown in phantom, may be painted or evaporated upon the surface of glass plate 18 to act as a conductive surface.

When the border 26 is laid over the screen 20, the removal backing is pro-scored preparatory to peeling away annular strips 26a, 26b, and 260. Thus, the internal area is first exposed to permit adhesion of the first dielectric layer C1 at its margins only. Note that the medial area of the layer C1 rests directly upon the conductive screen 20 (or the conductive coating 25, if employed), without being cemented thereto. Next, the dielectric layer C2 is secured at its marginal edges to the annular zone of the tape border 26 which has been exposed by stripping away the backing tab 26b. A surface of talc 28 may be dusted on the opposed faces of the layers C1 and C2 to maintain the middle zones spaced from each other in non-adhesive but abutting disposition. Thereafter, the dielectric layer C3 is marginally secured to the outer annular zone of the border tape 26 which has been stripped by peeling away the backing 26c. Again, a talc interface 30 may be dusted between the opposing faces of the layers C2 and C3.

The multilayers C1, C2 and C3 are each of 1 mil sheet vinyl fluoride (Tedlar) which has a high dielectric constant K=8.6. This material owes its high dielectric constant to intermingled, long polymeric chains of unsymmetrical links which result in an increase in 8 Also note that the layers C1, C2 and C3 are quite thin to provide multiple interfaces for the capture of pre-charged carriers without increasing the total thickness, 1. This results in an increase in & in addition to providing adequate insulation.

The layers 28 and 30 of powdered talc, whose average particle size is one micron when dusted on the interfaces, prmides a thin mono-particle layer which maintains the thin dielectrics separated by approximately 20,000 angstroms which is essentially abutting. Since talc is an inorganic material whose composition is primarily magnesium silicate, a very unsymmetrical molecule containing two hydroxyl radicals, it additionally provides a highly polar interface effective for trapping charge.

Laid directly over the layer C3 is a glass cover plate 32, preferably containing heavy elements such as lead or barium. This makes available a myriad oftraps to further enhance the effective dielectric constant. Opal glass is satisfactory for this purpose. The transparent or translucent cover plate 32 is secured to the frame 12 by a peripheral bezel 34 and allows for transmission of light from lamps 16 therethrough for illuminating objects on its surface.

One side of the high voltage D.C. source D is coupled to the conductive surface 20 by extending a cable lead 36 through a bored opening in the base plate 18. Approximately 7,500 to 15,000 volts are applied to the conductive surface B to supply the electric field E. It has been found that a trap potential or barrier of between 2 to 6 electron volts is produced in the interface zones 28 and 30.

From the foregoing description, it is apparent that there has been a deliberate attempt to avoid the use of adhesives in the multilayer zones C1, C2 and C3 so as to afford better interface isolation for trapping charge and hence to improve holding force. It is also possible to eliminate the talc interfaces and allow the adjacent layer surfaces to abut whereby the normal molecular roughness of the surface texture itself furnishes a spacing in the range of 200 angstroms. Still another mode of providing surface asperity is by deliberate mechanical abrasion of the surfaces in contradistinction to the relatively smooth molecular roughness of said surfaces. Thus, the interfacial polarization supplements the other polarization mechanisms to optimize charge carrier trapping. Measurements indicate far superior holding forces at the surface of the copyboard.

Although this invention has been described in considerable detail, such description is intended as being illustrative rather than limiting, since the invention may be variously embodied, and the scope of the invention is to be determined as claimed.

What is claimed is:

1. An electrostatic copyboard comprising means constituting a conductive surface,

a plurality of adjacently spaced dielectric layers superimposed in face-to-face disposition with each other, means for maintaining opposing surfaces of adjacent dielectric layers separated from each other by a spacing gap in the range of 200 to 20,000 angstroms to provide an interface for trapping charge,

said means constituting a conductive surface being in abutment with one of the outboard dielectric layers, and

means for applying a DC. high voltage to said means constituting a conductive surface, whereby enhanced electrostatic retaining forces are provided for articles in juxtaposition with the other of the outboard dielectric layers.

2. The copyboard of claim 1 wherein said means for maintaining the surface spacing comprises a dielectric powder.

3. The copyboard of claim 2 wherein said dielectric powder is talc having an average particle size in the range of one micron.

4. The copyboard of claim 1 wherein said means for maintaining the surface spacing constitutes rough surfaces on opposing faces of said layers.

5. The copyboard of claim 1 wherein the layer next adjacent the conductive surface is in intimate contact therewith.

6. The copyboard of claim 1 wherein the volume resistivity of the spaced layers is in excess of 10 ohm centimeters.

7. The copyboard of claim 1 wherein the dielectric constant of said layers is in excess of 2.

8. The copyboard of claim 1 wherein said conductive surface constitutes a metallic screen.

9. The copyboard of claim 1 wherein said conductive surface constitutes a coating.

10. The copyboard of claim 1 wherein each of said dielectric layers is of the thinnest cross-section as will substantially provide insulation against high voltage breakdown.

11. The copyboard of claim 1 including a glass cover plate superimposed over the other of the outboard dielectric layers.

References Cited UNITED STATES PATENTS 3,194,131 7/1965 Robinson 355-3 3,340,446 9/1967 Cox 3l7259 XR 3,359,469 12/1967 Levy et a1. 317-262 NORTON ANSHER, Primary Examiner R. L. MOSES, Assistant Examiner U.S. Cl. X.R. 3553, 113 

